Diagenesis and Reservoir Quality of the Oligocene Vedder Sandstone, of the Rio Bravo Oil Field, California

Abstract

The Rio Bravo oil field is located about 15 miles north-west of Bakersfield, CA. The zone of importance is the Vedder Sandstone, which is about 1,250 feet (380m) in thickness. The thin (<100ft, 30m) Miocene Rio Bravo sandstone, which unconformably overlies the Vedder, is included in the main Vedder reservoir. Burial depths range from approximately 10,750 feet (3,415m) to 12,450 feet (3,800m), with reservoir temperature at 120°C. The mineralogy and lithology of Oligocene sandstones of the Rio Bravo oil field were examined using a petrographic microscope and a scanning electron microscope equipped with and energy dispersive x-ray spectrometer (SEM-EDS) and a cathode luminescence imaging system (SEM-CL). The Vedder Sandstones are medium to fine-grain, subangular to subround, very poorly to well sorted, arkosic arenites and wackes. Accessory minerals of the Vedder Formation include biotite, muscovite, chlorite, glauconite, pyrite, zircon, zeolite, hornblende, rutile, phosphate, and apatite. The diagenetic features affecting reservoir quality of the Vedder Sandstones are similar among wells. Albitization occurs extensively along fractures in plagioclase and K-feldspar grains. Plagioclase shows varying degrees of alteration to clay or sericite. Biotite has been altered to chlorite and pyrite. Precipitation of cements include clays (kaolinite, chlorite, and illite and/ or mixed-layer illite/smectite or illite/chlorite), and carbonates. Kaolinite occurs as pore-filling cement, commonly associated with feldspar dissolution. Carbonates include calcite and dolomite. Calcite cement occurs within some through going fractures. Both calcite and dolomite have partially to completely replaced framework-grains. Porosity within the Vedder sands is controlled mainly by compaction and dissolution of framework-grains. Compaction decreased porosity through ductile grain deformation of shale clasts and micas, which commonly were squeezed into adjacent pores to form pseudomatrix. Rotation and slippage of grains and fracturing of brittle grains is also widespread. Dissolution of framework-grains created oversized and elongate pores, with the result that secondary intergranular porosity contributes significantly to overall reservoir quality.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015